Heretofore, various many types of such recliner-back tilting mechanisms have been proposed; one of them is disclosed in the Japanese Patent Laid-Open Publication No. Sho 53-95744, for example, according to which a base plate is fixed to the frame side face of the seat of a recliner, and an arm is fixed to the frame side face of the back, said arm being pivotably mounted on a shaft fixed to the base plate. In order to retain or free the arm against or for pivoting with respect to the base plate, the arm has formed in one end thereof an open circular hole on the entire inner circumference of which inner teeth are formed, and retainers in pair having outer teeth formed thereon which are in mesh with said inner teeth, are so disposed between guide faces for each of them on said base plate to be away from each other in symmetry with respect to the shaft. Furthermore, there is provided a rotary cam mechanism which permits said retainers to move toward each other to provide disengagement between said inner and other teeth, while permitting said retainers to move away from each other to make said inner and outer teeth engage with each other. An operating lever for control of said rotary cam mechanism is installed to the base plate by means of a spring for securing the lever in place.
In such prior-art recliner-back tilting mechanisms, however, although there are formed, as described above, teeth along the entire circumference of the circular opening in the arm end, all these teeth are identical and thus only some of them contribute to tilting of the back of a recliner. Further, simultanous engagement between the outer and inner teeth will gradually be unsuccessful because the teeth deteriorate due to the weight of the recliner back as well as to the repeated operation, resulting eventually in damage to the teeth.